UC-NRLF 


STHUCTl'iAL  TIUBIUS 

I.  Built-Up  Yellow  Pine  Timbers  Tested  for 

Strength. 

II.  i.iaximum  Spans   for   Joists  and    Rafters. 

National  Lumber  Manufacturers  Asso. 


c.- Forestry.  Main  Librar> 


vurj 


.-  forestry.  Maia  Library 
NATIONAL  LUMBER  MANUFACTURERS  ASSOCIATION  -WOOD  CONSTRUCTION  INFORMATION  SERVICE 

Series    E-2b. 


Built-up  Southern  Yellow  Pine  Timbers  Tested  for  Strength  1 


FIG.  1— SHOWING    METHOD  OF  APPLYING   LOADS  TO   BEAM    IN   TESTING    MACHINE. 


Built-up  structural  beams  have  an  advantage  over  solid  beams  i 
the  utilization  of  smaller  stock,  quick  and  easy  seasoning,  availability, 
and  the  possibility  of  avoiding  bad  checks,  shakes  and  other  defects.          ^^^     .  >;.^  _, 
Tests  made  at  the  Forest  Products  Laboratory  on  eleven  built-up  beams    (*  4  j    J  ? .  ^  •  ;.-4  ^*v 
of  southern  yellow  pine  indicate  that  from  the  standpoint  of  strength  r"^     ' 

there   is    no    marked   difference    between    built-up    beams    of    "dense" 

•i  •       11        »  /  it  i          i-  i     »  /  »     C'~-  '  T'"   C^   Ti  AGBVC' 

material   practically   free   from   defects    and    solid    beams    of      dense 
material  with  defects  limited  as  in  the  select  structural  grade  of  the    lt 
Southern  Pine  Association. 


T 

1 


Purpose 

These  tests  were  made  in  co-operation  with  the 
National  Lumber  Manufacturers  Association  to 
obtain  information  on  the  behavior  of  built-up 
beams  under  loads,  their  strength  properties  as 
compared  with  solid  structural  timbers  previously 
tested,  and  to  ascertain  the  advantages,  if  any, 
of  using  built-up  instead  of  solid  timbers.  Tests 
on  small  clear  specimens  cut  from  the  large  tim- 


bers were  made  to  furnish  data  for  comparison 
of  the  strength  properties  of  structural  timbers 
and  small  clear  specimens.  This  investigation  is 
preliminary  to  a  more  comprehensive  study  of 
laminated  and  built-up  beams  and  trusses. 

Character  and  Condition  of  Material  Tested 

Fifty-five  2-inch   by    12-inch  by   16- foot   planks 
of   commercial    southern    yellow    pine    were    used. 


l  Tests  made  at   the   U.  8.   forest  Products  Laboratory,  Madison,  Wis.,  in  co-operation  with  the  National  Lumber 
Manufacturers  Association,    Washington,   D.   C.—By   O.   E.    Heck,  Engineer  in  Forest  Products. 

This  is  a  progress  report   of  one  of  a  number  of   investigations     on    woods    commonly    used    for     construction 
purposes. 


51001)4 


Page  Two 


BUILT-UP   SOUTHERN    YELLOW   PINE    TIMBERS    TESTED    FOR    STRENGTH 


This  material  had  been  air-dried  and  had  about 
17  per  cent  moisture  content  at  the  time  it  was 
tested.  The  planks  were  plain  sawed  and  prac- 
tically free  from  defects,  but  about  one-half  the 
number  contained  season  checks,  some  of  which 
opened  up  considerably  after  assembly  in  the 
beams  and  before  testing.  A  few  of  the  planks 
were  somewhat  cross-grained,  but  not  seriously 
enough  to  preclude  their  use  for  this  series  of 
tests  on  beams  built  up  of  clear  stock. 

The  solid  beams  (previously  tested)  with  which 
comparison  is  made  were  limited  in  defects  as  in 
the  select  structural  grade  of  the  Southern  Pine 
Association  and  were  selected  by  a  representative 
of  the  laboratory. 

Construction  of  the  Beams 

Each  built-up  beam  consisted  of  five  planks 
bolted  together.  The  planks  composing  a  beam 
were  matched  by  comparing  ends,  material  of 
practically  the  same  quality  being  used  in  each 
beam,  and  were  surfaced  on  both  sides  in  order 
to  have  the  resulting  beams  of  uniform  dimensions 
and  all  p'anks  fitting  closely  together.  All  bolts 
and  the  holes  into  which  they  were  driven  were 
34-inch  in  diameter.  The  bolts  were  spaced  as 
indicated  in  Fig.  2.  Washers  1^4  inches  in  diam- 
eter were  used  under  the  heads  and  nuts  to  pre- 
vent crushing  of  the  outer  planks.  After  the 
beams  were  constructed  they  were  surfaced  on 
the  top  and  bottom. 

Method  of  Testing  Built-up  Beams 

The  built-up  beams  were  tested  in  accordance 
with  standard  practice  of  the  United  States  Forest 
Service1,  on  a  200,000-pound-capacity  Richie  test- 
ing machine,  which  is  provided  with  an  extension 
weighing  platform  (see  Fig.  1).  The  beams  are 
placed  on  two  knife-edge  supports  15  feet  apart, 
that  rest  on  the  platform  of  the  testing  machine. 
The  test  load  is  applied  at  the  third  points  of  the 
beam.  A  fine  wire  kept  taut  by  means  of  rubber 
bands  at  the  ends  is  strung  between  two  nails  15 
feet  apart  driven  midway  between  the  top  and  bot- 
tom faces  of  the  beam  and  vertically  above  the 
knife-edge  supports.  As  the  test  load  is  applied 
the  beam  deflects  and  the  steel  scale  which  is 
fastened  to  the  beam  midway  between  the  sup- 
ports moves  down  while  the  wire  does  not  change 
its  original  position.  The  distance  the  scale  moves 
relative  to  the  wire  indicates  the  amount  of  de- 


flection or  bending.  Deflection  readings  are  taken 
rt  suitable  increments  of  load.  In  this  instance, 
the  loads  were  read  every  2.000  pounds  until 
about  30,000  pounds  had  been  applied,  when  the 
weighing  beam  of  the  testing  machine  was  kept 
balanced  and  the  loads  read  for  every  1/10-inch 
deflection.  This  method  of  obtaining  the  load- 
deflection  curve  assisted  materially  in  locating  the 
elastic  limit  and  in  ascertaining  the  load  at  failure. 

Tests  of  Small  Clear  Specimens 

After  the  built-up  beams  had  been  tested,  small 
clear  test  specimens  were  sawed  from  each  plank 
in  the  beam  near  the  point  of  failure  and  tested 
according  to  the  laboratory's  standard  method  for 
testing  small  clear  specimens2.  The  nominal  sec- 
tional dimensions  were  1.6  by  2  inches.  These 
test  specimens  are  called  "minors." 


FIG     2— SPACING   OF   BOLTS   IN    BUILT-UP   BEAMS. 

Two  specimens  for  each  of  the  following  tests 
\vere  taken  from  each  plank  in  the  eleven  beams : 

Static  bending 

Impact  bending 

Compression  parallel  to  the  grain 

Compression  perpendicular  to  the  grain 

Hardness 

Shear 

Cleavage 

Tension  perpendicular  to  the   grain 

The  minors  sawed  from  the  solid  beams  with 
which  comparison  is  made  were  2  by  2  inches  in 
cross-sectional  dimensions. 


1  See    U.   S.   Forest   Service  Circular  38,  "Instructions    to   Engineers    of    Timber    Tests." 

2  See  U.  S.     Dept.   of  Agr.   Bulletin  556,  "Mechanical    Properties  of  Woods  Grown  in  the  United  States,"  by  J.  A. 
Newlin  and  T.  R.  C.  Wilson.    Can  be   obtained  from   U.  S.  Supt.  of  Documents,   Washington,  D.  C.    Price,  '  10  cents 


BUILT-UP   SOUTHERN    YELLOW   PINE    TIMBERS    TESTED    FOR    STRENGTH        Paye  Three 


//  BUILT-UP  Q£A*I3  ~No/nir>ot  dimensions  3~*/t£*/€'CJ*or  dense 
9     SOLID  -  ••  6'if!2"x/6  ' 59l*cr 

MINOR5-   3motl  c./*or  specimens    /.6»Z~x3O".    Cur    from 
sv/td    A  butlr-up    btoms. 


FIG.   3-STRENGTH    PROPERTIES  OF  SOLID  AND    BUILT-UP  TIMBERS   COMPARED  WITH    MINORS. 

(SOUTHERN  YELLOW  PINE.) 


Explanation  of  Experimental  Data 


In  Table  1  are  given  the  strength  properties  of 
the  built-up  beams  together  with  those  of  their 
minors.  The  adjustments  for  differences  in  mois- 
ture content  were  made  and  ratios  of  the  strength 
properties  of  the  large  beams  to  the  same  proper- 
ties of  their  minors  formed.  The  averages  of 
the  individual  strength  properties  were  computed 
and  these  are  used  as  a  basis  for  the  analysis  of 
the  results. 

Table  2  contains  data  on  the  solid  beams  similar 
to  those  found  in  Table  1  for  the  built-up  beams. 

Table  4  is  a  summary  of  Tables  1  and  2. 

In  Table  3  is  given  a  summary  of  the  results 
of  all  minor  tests  from  the  built-up  beams  as 
well  as  the  averages  of  these  resu'ts. 

Fig.  1  shows  the  manner  of  loading  the  beams 
during  test. 

Fig.  2  shows  the  spacing  of  the  bolts  in  the 
built-up  beams.  A  graphical  representation  of  the 
average  values  of  Tables  1,  2,  and  3  is  given  in 
Fig.  3.  The  insert  shows  graphically  the  ratio  of 
the  built-up  to  the  solid  beams.  Load-deflection 
diagrams  for  the  built-up  beams  are  shown  in  Fig. 
4.  Fig.  5  shows  a  loaded  beam  which  has  buckled 
sidevvise. 

How  the  Beams  Failed 

As  the  test  load  was  applied  the  beams  deflected 
gradually  until  the  elastic  limit  was  reached,  no 
visible  failure  occurring  until  after  this  load  was 


passed.  The  failures  are  classed  as  tension,  com- 
pression, horizontal  shear  and  sidewise  buckling. 
There  were  three  types  of  tension  failures :  cross 
grain,  brash,  and  splintering.  It  was  found  upon 
examination  of  the  individual  planks  after  test 
that  in  some  cases  there  were  compression  fail- 
ures which  were  not  visible  during  the  test  and 
no  doubt  the  first  compression  failure  noted  was 
not  in  every  case  the  first  failure  to  occur.  The 
horizontal  shear  failures  were  sudden  and  gave  no 
warning,  the  line  of  failure  generally  followed  the 
season  checks,  if  there  were  any. 

The  following  notes  on  the  tests  of  the  indi- 
vidual beams  will  assist  materially  in  understand- 
ing the  behavior  of  the  beams  during  test. 

Beam  No.  1. — The  first  failure  occurred  in  compres- 
sion at  a  load  of  42,000  pounds.  A  cross-grain  ten- 
sion failure  (slope  of  grain  1:12)  occurred  in  the 
middle  plank  of  the  beam  when  a  load  of  49,00r> 
pounds  had  been  applied.  The  test  was  continued 
until  the  load  reached  51,000  pounds,  at  which  point 
the  beam  buckled. 

Beam  No.  2 — A  faint  cracking  was  heard  at  a  load  of 
30,000  pounds.  A  cross-grain  tension  failure  (slope 
of  grain  1:19)  occurred  at  48,600  pounds.  The  test 
was  continued  to  50,000  pounds,  at  which  load  the 
beam  buckled.  Subsequently,  several  compression 
failures  occurred. 

Beam  No.  3 — A  horizontal  shear  failure  occurred  in 
plank  No.  1  at  a  load  of  25,260  pounds.  The  shear 
failure  was  influenced  by  the  presence  of  several 
season  checks.  Tests  on  the  small  specimens  indi- 
cate that  this  plank  had  a  modulus  of  elasticity  of 
2,200,000  pounds  per  square  inch,  which  was  much 


Page  Four          BUILT-UP   SOUTHERN    YELLOW   PINE    TIMBERS   TESTED    FOR   STRENGTH 


higher  than  for  the  other  planks  in  this  beam.  The 
slope  of  the  load-deflection  curve  was  changed  after 
the  horizontal  shear  failure  in  plank  No.  1  oc- 
curred. A  tension  failure  influenced  by  local  wavy 
grain  occurred  in  plank  3  between  support  and  load 
point  at  a  load  of  42,250  pounds.  The  test  was 
continued  and  compression  failures  were  first  no- 
ticed at  a  load  of  51,000  pounds.  The  maximum 
load  of  52,250  pounds  was  reached  when  several 
tension  failures  occurred  in  quick  succession. 

Beam  No.  4.— The  middle  plank  of  this 
beam  failed  by  horizontal  shear,  which 
was  probably  influenced  by  season 
checks,  at  a  load  of  53,590  pounds. 
The  test  w.as  continued  until  the  beam 
carried  57,220  pounds  when  several 
splintering  tension  failures  occurred 
with  a  simultaneous  lowering  of  the 
load. 

Beam  No.  5. — The  first  failure  was  by 
cross-grained  tension  in  plank  No.  1 
at  a  load  of  38,000  pounds  and  this 
was  followed  by  a  second  and  third 
tension  failure  in  the  same  plank  at 
a  load  of  42,000  pounds.  The  load 
then  dropped  to  39,200  pounds;  but 
the  beam  again  took  additional  load 
until  the  fourth  tension  failure  oc- 
curred at  41,000  pounds.  At  this  point 
the  first  compression  failure  was  no- 
ticed and  then  several  tension  failures 
occurred  in  rapid  succession. 

Beam  No.  6.— The  first  failure  was  by 
horizontal  shear  in  plank  No.  5  at 
37,360  pounds.  This  failure  was  in- 
fluenced by  season  checks.  The  load 
dropped  to  35,200  pounds  when  a 
compression  failure  was  noticed.  As 
the  test  continued  the  beam  took  ad- 
ditional load  until  a  brash  tension 
failure  occurred  at  40,400  pounds,  the 
load  dropping  to  38,300  pounds.  The 
load  was  again  increased  to  41,520 
pounds,  when  the  beam  buckled  and 
another  brash  tension  failure  occurred. 

Beam  No.  7. — The  first  failure  occurred 
by  compression  in  the  middle  plank 
of  the  beam  at  44,300  pounds.  When 
the  maximum  load  of  46,930  pounds 
was  reached,  three  tension  failures 
occurred  in  succession.  The  load  dropped  to  33,790 
pounds  and  the  test  was  discontinued. 

Beam  No.  8. — The  planks  in  this  beam  were  badly 
checked.  First  failure  occurred  by  tension  at  35.20J 
pounds  and  this  was  followed  by  a  second  tension 
failure  in  the  same  plank  at  36,300  pounds.  The 
scale  which  was  used  for  measuring  the  deflection 
flew  off  when  the  second  tension  failure  occurred, 
but  the  test  was  continued  to  a  maximum  load  of 
40,640  pounds. 

Beam  No.  9. — The  planks  composing  this  beam  were 
checked  but  not  to  such  an  extent  that  they  would 
be  expected  to  influence  the  failure.  The  first  fail- 
ure was  by  tension  in  the  middle  plank  at  a  load 
of  43,110  pounds.  The  load-deflection  diagram  was 
affected  very  little  by  the  first  failure.  Several 
compression  failures  occurred  as  the  test  was  con- 
tinued. The  maximum  load  occurred  at  49,690 
pounds,  when  a  horizontal  shear  failure  occurred  in 


plank  No.  1.  The  load  dropped  to  48,000  pounds 
and  then  increased  to  49,100,  when  tension  failures 
accompanied  by  compression  failures  occurred  rap- 
idly until  the  load  dropped  to  24,000  pounds  and 
the  test  was  discontinued. 

Beam  No.  10. — This  beam  was  made  up  of  planks 
having  spiral  grain  with  a  slope  greater  than  1  in 
20.  The  slope  of  grain  varied  considerably  through- 
out the  individual  planks.  Planks  which  had  a 


(SOUTHE 


4— LOAD-DEFLECTION   DIAGRAMS   FOR   BUILT-UP  BEAMS. 
-LOW    PINE,    NOMINAL    DIMENSIONS    BX    111-2    INCHES.    16    FEET    LONG.   13- 
FOOT   SPAN.    THIRD-POINT  LOADING.) 

slope  of  grain  greater  than  1  in  20  at  the  center 
were  often  straight-grained  at  the  outermost  fiber. 
Some  which  were  cross-grained  at  one  end  were 
straight-grained  at  the  other.  The  failures  of  the 
planks  in  this  beam  indicate  that  the  strength  was 
not  affected  as  much  as  would  be  expected  by  the 
presence  of  spiral  grain.  The  first  failure  was  in 
compression  at  a  load  of  38,000  pounds  and  was 
probably  due  to  local  wavy  grain.  The  second 
failure  was  also  compression  at  46,700  pounds  and 
this  was  followed  by  a  tension  and  a  compression 
failure  at  the  maximum  load  of  47,370  pounds.  The 
load  dropped  to  31,000  pounds,  from  which  it  again 
increased  until  it  reached  34,200  pounds,  when  sev- 
eral brash  tension  failures  occurred  in  succession. 

Beam  No.  11 — The  first  failure  was  at  a  load  of  47,- 
200  pounds,  when  compression  failures  occurred 
simultaneously  in  three  planks.  These  were  fol- 
lowed by  the  beam  buckling  sidewise  at  a  load  of 


BUILT-UP  SOUTHERN   YELLOW  PINE    TIMBERS   TESTED   FOR   STRENGTH 


Paije  Five 


49,250    pounds    without    any    tension    failures.      The 
test    was    discontinued    when    buckling    occurred. 

Solid  Beams. — The  solid  beams,  data  upon  which 
were  used  in  this  discussion  for  comparison,  were 
all  select  structural  grade  material  with  limited 
defects.  A  detailed  discussion  of  the  tests  of  this 
material  will  not  be  taken  up  in  this  article.  The 
strength  values  of  the  minors  taken  from  these 
beams  are  about  the  same  as  those  taken  from  the 


FIG.   S     FAILURE  OF  LOADED    BEAM. 

built-up  beams ;  consequently,  the  ratios  formed 
with  the  structural  sizes  should  give  comparable 
and  fairly  reliable  information  as  to  the  merits 
of  the  built-up  beams. 

Horizontal   Shear 

The  average  horizontal  shear  stress  developed 
in  the  built-up  beams  was  394  pounds  per  square 
inch.  The  range  of  results  is  from  335  to  473 
pounds  per  square  inch.  The  average  horizontal 
shear  stress  developed  in  the  solid  beams  was  396 
pounds  per  square  inch  and  the  range  of  results 
was  from  324  to  483  pounds  per  square  inch.  It 
is  interesting  to  note  in  built-up  beam  No.  4, 
which  developed  the  highest  shear  stress  and  also 
the  highest  fiber  stress,  that  the  horizontal  shear 
and  first  tension  failure  occurred  very  close  to- 
gether, and  in  the  same  plank. 

The  horizontal  shear  in  the  small  bending  speci- 
mens was  not  computed  since  the  ratio  of  depth 
to  length  was  such  that  a  relatively  small  part  of 
the  maximum  shear  was  developed.  The  shear 


ratio  shown  in  the  graphical  representation  of 
strength  properties,  Fig.  3  (also  see  Tables  1  and 
2),  was  formed  between  the  results  of  tests  on 
small  shear  test  specimens  and  the  computed 
shear  in  the  large  beams. 

Maximum  Fiber  Stress 

The  ratio  of  the  modulus  of  rupture  or  maxi- 
mum fiber  stress  of  the  built-up  beams  to  that  of 
the  small  test  specimens  is  72  per  cent ;  while  the 
same  ratio  for  the  solid  beams  is  66.3  per  cent 
based  on  average  values.  The  ratio  of  the  modu- 
lus of  rupture  of  the  built-up  to  the  solid  beams 
is  107  per  cent.  As  would  be  expected,  the  small 
specimens  gave  higher  values  for  modulus  of  rup- 
ture than  the  large  pieces  since  they  were  free 
from  defects  and  other  variables  that  influence  the 
strength  of  large  beams.  The  increase  in  the 
ratio  of  modulus  of  rupture  of  the  majors  to  their 
minors  in  the  case  of  the  built-up  beams  over  this 
ratio  for  the  solid  beams  is  slight  and  the  tests 
are  too  few  to  indicate  any  advantage  of  the  built- 
up  over  the  solid  beams. 

Fiber  Stress  at  Elastic  Limit 

The  elastic  limit  was  determined  from  the 
load-deflection  diagrams  by  finding  the  point  at 
which  the  deflection  increases  markedly  more  rap- 
idly than  the  load,  or  in  other  words,  the  point  at 
which  the  load-deflection  curve  deviates  from  a 
straight  line. 

The  average  fiber  stress  at  the  elastic  limit  for 
the  built-up  beams  was  6,160  and  for  the  solid 
beams  5,750  pounds  per  square  inch,  the  built-up 
being  7  per  cent  higher  than  the  solid  beams. 
(See  Table  1  and  Fig.  3.)  The  ratio  of  the  aver- 
age fiber  stress  at  elastic  limit  of  the  large  beams 
to  that  of  their  minors  is  74.6  per  cent  for  the 
built-up  and  70.7  per  cent  for  the  solid  beams. 

Modulus  of  Elasticity 

As  a  rule  the  stiffest  plank  in  a  built-up  beam 
takes  the  largest  share  of  the  load  and  the  planks 
of  lesser  stiffness  receive  proportionately  smaller 
shares.  The  stiffness,  or  modulus  of  elasticity,  of 
the  planks  in  a  built-up  beam  may  vary  under 
extreme  cases,  as  much  as  100  per  cent  or  even 
more. 

The  large  number  of  tests  made  at  this  labora- 
tory on  small  clear  specimens  have  shown  that  a 
piece  of  high  modulus  of  elasticity  normally  de- 
flects farther  to  the  elastic  limit  and  to  maximum 
load  than  a  less  stiff  piece.  This  being  true,  it  is 
apparent  that  the  built-up  beam  is  most  likely  to 
fail  in  a  plank  of  low  modulus  of  elasticity  rather 
than  in  the  stiffer  plank  even  though  the  latter 
carries  a  larger  share  of  the  load.  This,  however, 
is  not  true  of  defective  material,  for  the  first  fail- 
ure will  usually  occur  in  the  most  defective  timber. 


Page  Six 


BUILT-UP   SOUTHERN    YELLOW   PINE    TIMBERS   TESTED    FOR   STRENGTH 


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BUILT-UP   SOUTHERN    YELLOW   PIXE    TIMBERS    TESTEDFOR 


Page  Nine 


The  strength  of  the  built-up  beam  depends  upon 
the  deflection  the  defective  piece  takes  at  failure. 

The  modulus  of  elasticity  of  the  built-up  beams 
is  16.2  per  cent  higher  than  the  average  of  the 
small  test  specimens,  while  for  the  solid  beams 
this  ratio  is  17.8  per  cent  higher. 

Moisture  Distribution 

Determinations  for  moisture  distribution  show 
that  there  was  very  little  variation  in  moisture 
content  throughout  the  sections  of  the  built-up 
beams.  In  solid  air-dry  beams  in  structural  sizes 
it  is  not  uncommon  to  find  that  the  inner  portion 
of  the  section  has  a  moisture  content  10  per  cent 
higher  than  the  outer  portion. 

Where  Built-up  Beams  Fail 

The  opinion  has  often  been  expressed  that,  when 
two  or  more  timbers  or  planks  are  used  together 
and  loaded  so  as  to  deflect  equally,  the  stiffer 
pieces  will  take  the  greater  load  and  will,  there- 
fore, fail  before  the  less  stiff  pieces.  It  is  true 
that  the  stiffer  pieces  will  take  the  greater  load. 
Previous  tests,  however,  have  demonstrated  that 
dense,  stiff  pieces  usually  deflect  farther  to  the 
elastic  limit  and  to  failure  than  pieces  of  lower 
density  and  stiffness.  It  is  quite  evident,  there- 
fore, that  beams  built  up  of  clear  planks  will  tend 
to  fail  in  the  less  stiff  rather  than  in  the  stiffer 
planks. 

The  Forest  Products  Laboratory  has  made  no 
tests  with  defective  built-up  beams  to  ascertain  if 


the  defects  in  the  component  planks  can  be  stag- 
gered in  the  beam  and  the  latter  so  fastened  to- 
gether that  results  will  be  comparable  to  results 
on  solid  beams  containing  the  same  number  and 
size  defects  similarly  located.  Neither  have  any 
tests  been  made  to  determine  if  laminations  will 
act  independently  as  individual  beams,  each  break- 
ing at  its  particular  defect,  the  net  results  being 
practically  equivalent  to  having  all  the  defects  at 
the  same  point  in  a  solid  beam.  Such  tests  would 
be  of  value  in  making  a  further  study  of  built-up 
construction. 

TABLE   4. -SUMMARY   OF    AVERAGE    RESULTS    IN 
TABLES  1  AND  2  ON   BUILT-UP  BEAMS 

Ratio   (in   Per  Cent)  of  Structural  Sizes  to  Minors 


Fiber  stress    Modulus     Modulus 
at  elastic  of  of 

limit          rupture    elasticity 


Built-up     ...........     74.6          72.0         116.2 

Solid    (6   by    12-inch)     70.7          66.3         117.8 


Shear  1 

26.3 
34.1 


Ratio   (in   Per  Cent)   of  Built-up  to  Solid  Beams 
107.0        107.0         96.5        100.0 

Ratio    (in   Per   Cent)   of   Fiber   Stress   at   Elastic   Limit 
to   Modulus   of  Rupture 

Structural  Minors 

Built-up     .........................     72.5  70.2 

Solid    (6   by    12-inch)  ......  ........     72.2  68.0 

l   Ratio  of  computed  horizontal  shear  in   built-up   beams 
to    result!   obtained   on    standard   shear   specimens. 

U.    S.    Department   of    Agriculture, 

Forest    Service, 

Forest    Products    Laboratory, 

Madison,    Wisconsin 


Engineering  Bureau, 
Chicago,  Illinois. 
March  1,  1921. 


NATIONAL   LUMBER   MANUFACTURERS    ASSOCIATION. 


• 


**>. 


NATIONAL  LUMBER  MANUFACTURERS  ASSOCIATION  -WOOD  CONSTRUCTION  INFORMATION 


International   Building,  Washington.  D.  C. 


Harris  Trust   Building.  Chicago. 


Sept.  1st,  1922 


Page  One 


MAXIMUM  SPANS  FOR  JOISTS  AND  RAFTERS* 


The  following  tables  provide  a  handy 
means  of  determining  the  maximum  clear 
spans  for  wood  joists  and  rafters.  They 
are  based  upon  a  wide  range  of  strength 
values  and  cover  ordinary  load  conditions. 

The  span  length  should  be  limited  by 
deflection  to  prevent  cracks  where  ceilings 
are  covered  with  some  hard,  inelastic  ma- 
terial such  as  plaster.  Where  ceilings  are 
not  so  covered  and  where  a  small  amount 
of  sag  or  spring  is  not  objectionable  the 
span  length  may  be  determined  by  the 


bending  strength  of  the  member  instead  of 
by  its  stiffness. 

All  spans  given  in  these  tables  are 
based  on  the  actual  sizes  of  lumber. 

When  the  allowable  stresses  for  timber 
are  not  prescribed  in  the  local  building  code 
use  the  values  given  below.  They  are  taken 
from  the  recommendations  of  the  Forest 
Products  Laboratory,  Department  of  Agri- 
culture, at  Madison,  Wisconsin,  that  were 
officially  adopted  by  the  American  Society 
for  Testing  Materials  and  the  American 
Railway  Engineering  Association. 


ALLOWABLE  UNIT  STRESSES  FOR  STRUCTURAL  TIMBER 

(Pounds  Per  Square  Inch) 


Species  of  Timber 

Modulus  of 
Elasticity 

BENDING 

COMPRESSION 

Stress  in 
Extreme 
Fibre 

Horizontal 
Shear 
Stress 

Parallel  to 
Grain,  "Short 
Columns" 

Perpendicu- 
lar to 
Grain 

Cedar  Western  Red 

1,000,000 
800,000 
1,000,000 
1,400,000 
1,600,000 

1,500,000 
1,200,000 
1,000,000 
1,200,000 
1,400,000 

1,100,000 
1,300,000 
1;600,000 
1,100,000 
1,500,000 

1,600,000 
1,500,000 
1,000,000 
1,000,000 
1,200,000 

1,300,000 
1,200,000 
800,000 
1,300,000 

900 
750 
950 
1,300 
1,600 

1,300 
1,100 
900 
1,100 
1,300 

1,000 
1,200 
1,500 
1,000 
1,400 

1,600 
1,300 
900 
900 
1,100 

1,200 
1,100 
750 
1,200 

80 
70 
90 
100 
100 

90 
85 
70 
100 
75 

70 
100 
150 
100 
125 

125 
105 

85 
85 
85 

70 
85 
70 
95 

700 
550 
800 
1,100 
1,200 

1,000 
800 
700 
800 
900 

700 
1,100 
1,200 
800 
1,000 

1,200 
1,000 
750 
750 
800 

1,000 
800 
600 
1,000 

200 
175 
300 
350 
350 

300 
275 
150 
300 
300 

300 
325 
500 
350 
500 

350 
300 
250 
250 
300 

250 
250 
175 
300 

Cedar  Northern  White 

Chestnut 

Cvpress                     .    . 

Douglas  Fir  (No.  1  Struct.)  
Douglas  Fir  (No.  2  Struct.) 

Douglas  Fir,  Rocky  Mt.  Region  
Fir,  Balsam               

Gum,  Red                     

Hemlock,  Western  

Hemlock  Eastern 

Larch,  Western 

Maple,  Sugar  or  Hard..        .    .    . 

Maple,  Silver  or  Soft  

Oak  White  or  Red 

Pine,  Southern  Yellow  (Dense)  
Pine,  Southern  Yellow  (Sound) 

Pine,  Eastern  White  

Pine  Western  White 

Pine  Norway 

Redwood     

Spruce,  Red,  White  or  Sit  kn       

Spruce,  Engelmann  

Tamarack,  Eastern 

if  Prepared  by  Richard   G.   Kimbell,   primarily  ai   a   service  to   Building   Official*. 


Maximum  Joist  and  Rafter  Spans 


Page  Two 


AVERAGE  WEIGHTS  OF  VARIOUS  MATERIALS 


(For  use  in  determining  Dead  Loads.     These  weights  ware  uisd  in  obtaining 
Span  Lengths  which  appear  in  the  tables.) 


Joists: 


Nominal  Size 

2  x    4  ............................................  1%  x 

2  x    6  ............................................  1%  x 

2  x    8  ............................................  \%  x 

2  x  10  ............................................  1%  x 


Actual  Size  Wt.  per  Lin.  Ft. 

3%  ....................................................  1.6  Ibs. 

5%  ....................................................  2.5  Ibs. 

7Y2  ....................................................  3.4  Ibs. 

91A  ....................................................  4.3  Ibs. 


2  x  12 

2  x  14 

3  x    6 
3  x    8 
3  x  10 
3  x  12 


\%  x 


5.2  Ibs. 


\y%  x  13^  ....................................................  6.6  Ibs. 

2ys  x  5%  ....................................................  4.2  Ibs. 

2%  x  7}4  ....................................................  5.7  Ibs. 

2%  x  9J^  ....................................................  7.2  Ibs. 

2%  x  \\Yz  ....................................................  8.8  Ibs. 


3  x  14  ............................................  2%  x  13H  ....................................................  10.3  Ibs. 

4  x  6  ............................................  3%  x  5%  ..............  :  .....................................  5.6  Ibs. 

4  x  8  ............................................  3%  x  m.  ....................................................  7.8  Ibs. 

4  x  10  .....                                   ....3%  x  9J^  .....                                             ,  9.8  Ibs. 


Finished  floor 2.5  Ibs.  per  sq.  foot 

Rough  floor 2.5  Ibs.  per  sq.  foot 

Sheathing 2.5  Ibs.  per  sq.  foot 

Plaster 10.0  Ibs.  per  sq.  foot 


Roofing: 

Group  I — 

Shingles 2.5    Ibs.  per  sq.  foot 

Copper  sheets 1.5    Ibs.  per  sq.  foot 

Copper  tile 1.75  Ibs.  per  sq.  foot 

Three  ply  ready  roofing 1.00  Ibs.  per  sq.  foot 

Group  II — 

Five  ply  felt  and  gravel 7      Ibs.  per  sq.  foot 

Slate,  3/16  inch 7J4  Ibs.  per  sq.  foot 

Roman  tile — new  style — 1  part 8      Ibs.  per  sq.  foot 

Spanish  tile — new  style — 1  part 8      Ibs.  per  sq.  foot 

Ludowioi  tile 8      Ibs.  per  sq.  foot 


Mailmnm  Joist  and  Rafter  Span* 


FLOOR  JOIST  SPANS  (30  Pound  Load) 


Page  Three 


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Maximum  Joist  and  Rafter  Spans 


CEILING  AND  ATTIC  JOIST  SPANS 


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Maximum  Joist  and  Rafter  Spans 


RAFTER  SPANS  (30  Pound  Load— Group  I  Covering) 


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